1. Field of the Invention
The invention relates to a system for spraying a powder coating product, in particular an electrostatic spraying system for coating objects with a powder material, for example a thermofusible powder paint.
The invention relates more particularly to an improvement seeking to eliminate lumps of powder that can form and be entrained in the flow of air-powder mixture.
The invention also relates to a rotary powder sprayhead which preferably includes ionizing means for electrically charging the powder sprayed toward the item to be coated.
2. Description of the Prior Art
To guarantee a high deposition efficiency in the application of a powder coating product, such as a thermofusible product, to an object to be coated, it is necessary for the powder sprayed toward the object to be electrically charged, the object being at least weakly electrically conductive and grounded. The powder particles can be charged by a triboelectric effect but it is more usual to employ electrostatic charging means constituted by one or more electrodes situated in the vicinity of the air-powder mixture ejection orifice. A high voltage is applied to the electrode to charge the powder particles.
Some prior art systems use a sprayhead in the form of a bowl or disk having at least one rotary part which can be driven in rotation at a relatively high speed, for example by a pneumatic turbine. An annular space in which the air-powder mixture flows is defined inside the sprayhead, which causes the air-powder mixture to rotate and communicates to the powder particles centrifugal energy which creates a cloud of powder ejected toward the object. A ring of air ejector orifices all around the rear of the sprayhead directs the cloud of powder in the forward direction, i.e. toward the object to be coated. The ionizing electrode is generally combined with the rotary sprayhead. For example, it can be a metal disk forming the front face of the sprayhead with the edge of the disk in the immediate vicinity of the annular air-powder mixture ejection orifice defined at the periphery of the sprayhead. The latter incorporates an axial cavity in which an air-powder mixture feed pipe terminates.
A system of the above kind is generally satisfactory in operation, but under some unfavorable operating conditions lumps of powder are ejected and deposited on the part to be coated. After curing, these lumps cause appearance defects which necessitate reworking of the part. This reworking is difficult and costly.
There are essentially two kinds of lumps. One kind has no particular shape and consists of specks made up of powder granules. These specks can form under some unfavorable hygrometric conditions. They can be caused by the onset of polymerization of the powder due to heating. They are likely to be found in great numbers if the powder that is not deposited on the objects to be coated is recycled. Other lumps have a more specific shape. They are small flat lumps of powder and are usually referred to by the person skilled in the art as “eggshells”. Their formation is attributed to deposits of powder on the walls of the pipes in which the air-powder mixture flows. These deposits separate and break up from time to time, giving rise to the “eggshells”.
In-depth studies have identified the key points in a powder spraying installation that can influence the formation of lumps of powder in general and “eggshells” in particular.
Thus it has been shown that there is a relationship between the diameter of the air-powder mixture pipes and the rate of formation of the “eggshells”. Other things being equal, it is preferable to use small diameter pipes to reduce this rate. It has also been shown that it is preferable to fluidize the powder in the storage tank for some time before beginning to spray the powder. Thus ventilating the storage tank for 24 hours before use significantly reduces the proportion of “eggshells”. Finally, careful grounding of the storage tank also has a significant influence on the proportion of “eggshells”.
In all cases it is obviously advantageous to minimize the length of the pipes in which the air-powder mixture flows.
Although the above precautions reduce the rate of production of lumps, and more particularly of “eggshells”, it has been shown that completely preventing them from being formed is not possible. This is why the basic idea of the invention is to destroy such lumps in the air-powder mixture before it is ejected toward the part to be coated, more particularly in the sprayhead including a rotating part which can be driven in rotation, exploiting the centrifugal force applied to the air-powder mixture in the narrow flow space in the sprayhead.